As well known, in the paper converting industry a variety of types is used of machines and of processes for making paper tissues, paper towels and similar articles in stacks of interfolded sheets of a certain height.
The stacks are obtained folding the sheets and simply overlapping them on one another or interfolding them, i.e. at each fold a wing of the previous sheet and a wing of a next sheet of the stack engage with each other. This way, when drawing a sheet from a package, at the moment of the use, also a wing of a next sheet of the stack is dragged up to protruding from the package, with subsequent practical utilization for certain types of users. Among possible interfolding ways the L-type, with 2 folds (single fold), or the Z or W types, respectively with 3 and 4 folds (multi fold), are known.
After a step of separation of a pack from the stack, once achieved a measured height of the pack, a step follows of conveying the packs or logs for following treatment and packaging.
The stacks of folded or interfolded sheets from which a log is formed have a length that depends on the width of the starting web, normally about 2 metres and more.
For example, in case of logs of interfolded products exiting from the head of an interfolding machine, according to the type of interfolded product and then to the final use, said log can be conveyed to a transversal cutting machine forming small packages of predetermined length and then forwarding them to a binder that puts them in wrappers tailored for the users. With this solution, normally, the so-called “facial tissue” products are packaged.
Alternatively, the products can be distributed as banded packs or clips, as is the case of “hand towels”, which can be put in special fixed distributors with standard shape. In this case, the whole log of interfolded product, normally, is at first compressed and then banded by a web that covers it around, creating a tubular wrapper. The banded log is then cut off into single small packages or clips that are already banded by a corresponding cut tubular wrapper portion.
In the latter case, for carrying out the banding step, banding machines are used located downstream of the interfolding machine. In particular, the log of interfolded sheets produced and exiting from the interfolding machine can be carried directly into the banding machine by means of an appropriate conveying system. The log moves towards the banding machine travelling on a conveyor belt. A known system of this type is for example described in U.S. Pat. No. 6,865,861.
At the entrance in the banding machine the log meets a portion of path set between two overlapped conveyor belts that gradually decrease their relative distance in the vertical direction in order to compress the log. Once ended the compression step or contemporaneously with it, the banding is carried out by means of a packaging sheet unwound from a web, as described in U.S. Pat. No. 6,865,861.
In other solutions, instead, there is a movable upper support that compresses the log transported by the lower conveyor belt at a compressing position, reaching the same object of pressing the material of the log and of reducing its height before packaging. For example, a machine of this type is described in EP1636093.
Once the log has been conveyed completely in the machine a pusher brings it forward against a packaging sheet stretched along all the length of the machine. The packaging sheet can be of different material and in any case is automatically unwound from a roll and stretched in front of the log as an obstacle. Owing to the pusher that pushes the log against the stretched sheet a kind of self-packaging is carried out, i.e. the log is wound by the sheet for ¾ of its longitudinal sides.
A step follows of closing the packages by glueing. In particular, in one of the possible configurations of the state of the art, the roll from which the packaging web is unwound can be arranged in line to the banding machine by means of a special unwinder. The packaging web is located in stretched position in front of the log by pulling means. Then the log is pushed horizontally by a pusher in a zone between fixed upper and a lower plates meeting said paper in order to cross it and to allow it to band the log on three sides. In this case, being the unwinder of the paper located in line with the banding machine, the glue is spread only after banding the log with the packaging web for ¾. In this way glue is distributed along all the length of the upper wing of the web. Then, plates that move adjacent to the log close the wings. The banded log are pushed forward against one another exiting from the channel defined by the two fixed plates. The pushing actions in turn of the other packages assist the compression of the glued wings and thus the closure of the package.
Alternatively, it is possible to feed the paper horizontally, i.e. longitudinally with respect to the log. These solutions where unwinding the paper is carried out laterally, are more technologically advanced. In this case a single fixed glue gun is enough for the distribution of the glue when unwinding the paper along all the width of the banding machine. The paper is unwound by means of two couples of belts (an upper and a lower couple) that grip the paper and pull it for all the width of the banding machine. Said structural simplification provided by this solution requires however other structural complications and limits. In fact, the use of a single fixed glue gun (normally located between the paper path and the lower plates), allows a distribution of the glue when moving the paper in front of the log and then before the banding for ¾ of the lateral sides as described in the previous solution. This causes a loss of glue previously spread along the length of the paper being unwound when the paper is wrapped around the log during the packaging steps, in particular, in the motion through a narrow gap through which the paper moves pulled by the belts through the plates that form the introduction channel.
In order to solve this problem, in the prior art, a translatable drawing mechanism is provided. In this way, once wrapped the log by completion of the pushing action against the web, this drawing mechanism withdraws creating automatically an upper gap to prevent glue from adhering to mechanical parts of the machine.
Since the drawing mechanism engages with the log after that it has been wrapped around three sides, this prevents that the glue spread along the length of the paper dirties the components of the machine, and, in particular, the upper or lower plates, and that the film of glue is spoiled, causing a faulty adhesion of the two wings on the fourth side.
The use of the drawing mechanism, however, is a considerable structural complication. In fact, in addition to higher costs for making the machine, it causes a limitation on the maximum width of the machine, since a drawing mechanism too long would have problems for low stiffness. Furthermore, it requires a strong limitation to productivity necessary to the drawing mechanism for withdrawing and allowing the following closure of the wings.
On the other hand, the removal of the drawing mechanism has the problem of the glue dirty caused by the motion of the glued paper between the plates.